The present invention relates to a reference voltage generating circuit, particularly, to a reference voltage generating circuit used in an integrated circuit and, more particularly, to a reference voltage generating circuit in which arbitrary temperature dependency can be obtained over a wide output voltage range.
As a reference voltage generating circuit for outputting a predetermined reference voltage, a Widlar bandgap reference voltage circuit like the one shown in FIG. 8 is known (P. R. GRAY & R. G. MEYER, Analysis and Design of Analog Integrated Circuits, Chapter 4).
As shown in FIG. 8, this reference voltage circuit comprises a Widlar current mirror circuit made up of transistors Q1 and Q2 and resistors R1, R2, and R3. The operating point of the reference voltage circuit is determined by a feedback loop so as to obtain an output voltage Vout equal to the sum of a base-emitter voltage VBE of a transistor Q3 and a voltage proportional to the difference between the base-emitter voltages of the two transistors Q1 and Q2.
In other words, the output voltage Vout can be regarded as the sum of the base-emitter voltage of the transistor Q3 and a voltage drop across the resistor R2. Since the collector current of Q2 is almost equal to the emitter current, the voltage drop across R2 is the product of a voltage drop across R3 and (R2/R3). The voltage drop across R3 is equal to the difference between the base-emitter voltages of Q1 and Q2.
The output voltage Vout and its temperature coefficient are therefore given by ##EQU1## where N is the constant determined by an emitter area ratio of Q1 and Q2, VT is the thermal electromotive force, and VT=kT/q (k: Boltzmann's constant, T: absolute temperature, and q: electron charge).
In equation (2), .differential.VBE (Q3)/.differential.T&lt;0, and .differential.VT/.differential.T=k/q&gt;0 hold. Accordingly, in the Widlar bandgap reference voltage circuit, an arbitrary temperature coefficient including 0 can be realized by properly selecting R2, R3, and N.
In the Widlar bandgap reference voltage circuit, however, the range of Vout is as narrow as about 1.0 to 1.2 V because Vout is the sum of VBE (about 0.8 V) at Q3 and KVT (about 0.2 to 0.4 V).
To the contrary, for example, Japanese Patent Laid-Open No. 63-234307 (to be referred to as reference 1 hereinafter) discloses a bias circuit in which the output voltage Vout has an arbitrary temperature coefficient and which can output a voltage lower than the voltage of the Widlar bandgap reference voltage circuit.
As shown in FIG. 9, this bias circuit comprises a bandgap type constant current source 70 for outputting a current Is proportional to a thermal electromotive force VT, a current mirror circuit 80 made up of transistors Q1 and Q2 and resistors R1 and R2, a transistor Q3 which receives the current Is at the base, a transistor Q4 having a collector connected to the collector of the transistor Q2 of the current mirror circuit 80 and a base connected to the collector of the transistor Q3, a resistor R4 connected between the base and emitter of the transistor Q4, and a resistor R5 connected between the emitter and reference voltage of the transistor Q4. The output voltage is obtained by the collector terminal (Vout1) of the transistor Q3 or the emitter terminal (Vout2) of the transistor Q4.
The two output voltages Vout1 and Vout2 are given by ##EQU2## where N is the emitter area ratio of transistors QS1 and QS2, and VF is the base-emitter voltage of an NPN transistor.
The bias circuit in reference 1 comprises the two output voltage terminals Vout1 and Vout2. Vout1 outputs a voltage of VF or less, and Vout2 outputs a voltage of VF to 2VF. For this reason, a continuous voltage cannot be obtained by one terminal.
Partially differentiating right- and left-hand sides by the absolute temperature T yields ##EQU3## This means that adjusting the temperature coefficient of either one of Vout1 and Vout2 shifts the other temperature coefficient by .differential.VF/.differential.T. In the bias circuit in reference 1, therefore, the temperature coefficients of the two output voltages Vout1 and Vout2 cannot be made to coincide with each other.
Japanese Patent Laid-Open No. 58-97712 (to be referred to as reference 2 hereinafter) discloses a reference power supply circuit having an arbitrary temperature coefficient and a wide output voltage range.
As shown in FIG. 10, in the reference power supply circuit in reference 2, resistors R95 and R96 are respectively connected between the base and collector of a transistor Tr5 and between its base and emitter. The collector of the transistor Tr5 is connected to the base of a transistor Tr3. The emitter of the transistor Tr3 is connected to the emitter of the transistor Tr5 via a resistor R94. The emitter of the transistor Tr5 is also connected to a common terminal GND.
The collector of the transistor Tr5 receives a small current from the collector of a transistor Tr2 constituting a current mirror circuit 90.
The collector of the transistor Tr5 is further connected to the base of a transistor Tr4. The emitter of the transistor Tr4 is connected to the emitter of the transistor Tr5, and its collector is connected to Vcc via a resistor R93.
In the reference power supply circuit in reference 2, the base voltages of the transistors Tr3 and Tr4 are generated by a circuit (VBE multiplying circuit) made up of the resistors R95 and R96 and the transistor Tr5. Accordingly, an output voltage VX becomes unstable owing to variations in hFE of Tr5 caused by variations in manufacturing process or temperature.
In addition, the output voltage VX is influenced by variations in Vcc because it is equal to the difference between the external power supply voltage Vcc and the voltage across the resistor R93.
Japanese Patent Laid-Open No. 60-96006 (to be referred to as reference 3 hereinafter) discloses a reference voltage circuit capable of easily setting an arbitrary temperature coefficient and an arbitrary output voltage value.
As shown in FIG. 11, in this reference voltage circuit, the base voltages of transistors Q21 and Q22 are generated by resistors R21 and R22 connected to the emitter path of a transistor Q23. The collectors of the transistors Q21 and Q22 are connected to a current source by a current mirror circuit made up of transistors Q24 and Q25. The base of the transistor Q23 is connected to the collector of the transistor Q24. The emitter path of the transistor Q22 is connected to a resistor R23.
A reference voltage Vref is extracted from a resistor R24 connected to a power supply source formed from a transistor Q26 constituting the current mirror circuit together with the transistors Q24 and Q25.
The reference voltage circuit can arbitrarily set its temperature coefficient by adjusting the operating current density ratio of the transistors Q21 and Q22 and the ratio of the resistors R21 and R22. Further, this circuit can obtain an arbitrary reference voltage value by adjusting the ratio of the resistors R23 and R24.
The portion determining the reference voltage Vref serves as a frequency multiplier for the transistor Q22. For this reason, it is difficult to compensate for variations in base current caused by variations in hFE of the transistor Q22, similarly to the reference power supply circuit in reference 2.
As described above, in the prior art, no arbitrary temperature coefficient including 0 can be obtained in a wide range. Besides, the output voltage is influenced by variations in external power supply voltage Vcc.